During the Proterozoic era, oxygen produced by photosynthesizing
bacteria collects in the oceans and then in the atmosphere. Iron present in exposed
rocks reacts with this oxygen, and rust colors the planet red. While oxygen-loving
organisms evolve in these changing conditions, oxygen-intolerant anaerobes survive
only in airless pockets.

Soft-bodied animals, large enough to be seen by the naked eye,
evolve around 900 mya and are numerous by 650 mya. By roughly 550 mya, however, they
are scarce. Because so little fossil evidence of animals of this age has been discovered,
scientists are not certain whether these earliest animals were wiped out through an
extinction event at the end of the era or whether they evolved into the hard-shelled
life that follows.

2,500 mya:Banded iron formations

Banded iron formations (BIFs) are sedimentary rocks deposited
extensively on the ocean floor over hundreds of millions of years. Their alternating
rust- and gray-colored bands are evidence that oxygen is being produced in the oceans
at least as early as 2,500 mya.

Air apparent: The oxygen revolution (2,500-1,900 mya)

As more and more cyanobacteria (blue-green bacteria) spread
across Earth, the oxygen waste they produce through photosynthesis proves toxic
to most other microbes. In fact, only those sheltered in oxygen-poor
habitats like the murky depths and those with genetic mutations that somehow
enable them to tolerate oxygen survive.

How can we be sure that oxygen was present so long ago? Evidence
lies in banded iron formations -- the primary source of today's global iron ore
supply. From the 600 trillion tons of iron ore present today, we know there was
plenty of iron in Precambrian waters.

Iron normally stays dissolved in seawater; it falls out of
solution when it comes in contact with oxygen. The fine, bright orange particles
that settle on the ocean floor are the product of a chemical reaction: rust. The
alternating layers of rust-colored and gray deposits suggest oxygen production
fluctuated over time.

As underwater chimneys called deep-sea vents release dissolved
iron into Precambrian waters, oxygen is used up as quickly as it is produced. Once
the iron supply is exhausted, however, oxygen begins escaping the seas into the open
air. Evidence of a buildup of atmospheric oxygen first appears in rock layers
2,200-1,900 million years old, during which time most of the planet's exposed
surface rusts.

By 1,900 mya, oxygen composes about three percent of the
atmosphere's vital gases. This level will later rise to present-day levels of
21 percent. As the supply of oxygen increases and organisms increasingly tap
energy from it, cells grow larger and divide more quickly.

2,000 mya:Oxidation produces "red beds"

The reaction of oxygen with iron in exposed rocks begins producing "red beds"
of rust.

1,900 mya:Oxygen levels reach three percent

Oxygen now composes three percent of the atmosphere, still less than
one-fifth of the present-day level.

1,800 mya:Oldest eukaryote fossils

Though eukaryotes are thought to have evolved much earlier,
the oldest definitive fossils date to 1,800 mya. Acritarchs are probably single-celled
algae in a resting stage of their life cycle that have grown tough outer coats. They are
spherical in shape and float at or near the surface of the water.

1,200 mya:True algae

The oldest multicellular algae fossil dates to 1,200 mya. At this time,
biological diversity increases greatly, with algae, fungi, animals, and plants diverging
from their common eukaryote ancestors.

The oldest fossil evidence of multicellular animals, or metazoans, are
burrows that suggest smooth, wormlike creatures live 900 mya or more. Found in rocks in
China, Canada, India, and elsewhere, the imprints of these soft-bodied creatures reveal
little else besides their basic shapes.

The riddle of Ediacara (900 mya)

Trace fossils of wormlike creatures suggest complex multicellular
life evolved as far back as 900 mya. More extensive Precambrian finds, particularly
those discovered in Australia's Ediacara Gorge in rocks 565 million years old, include
fossil imprints of creatures that resemble everything from leafy fronds to jellyfish.
So what exactly were these highly diverse Precambrian creatures? Plants, animals, or
of another kingdom entirely?

Nearly all scientists today agree that they were soft-bodied
marine-dwelling animals. Their discovery confirmed what many scientists had long
hypothesized but could not prove: that skeletonized creatures of the Cambrian period,
long thought by scientists to be the first multicellular animals, had predecessors.

Debate continues, however, as to what became of the first animals.
While many scientists think that they subsequently evolved into new forms of animals
that continued into the Cambrian period, others say that they were wiped out by
extinction.

800 mya:Major glaciation period begins

The first of four global ice ages sets in. While cold-loving
microbes evolve and thrive in the frigid waters and snow, repeated glaciation
factors heavily into future extinctions of other organisms.

Did you know?

An ice age is characterized by intervals of repeated glacial
advances and retreats, not a single and prolonged deep freeze. Because glaciers
reflect solar light rather than absorb it, temperatures at the planet's surface
continue to drop, forming even larger glaciers. What causes glaciers to form in
the first place? Scientists say changes in atmospheric composition or fluctuations
in Earth's orbit and spin axis are the most likely triggers.

700 mya:Breakup of Rodinia supercontinent

As tectonic plates shift, the Rodinia supercontinent begins to break
apart into the smaller continents of Laurentia, Baltica, Siberia, and Gondwana.

600 mya:Protective ozone layer in place

With the exception of bacteria, life on Earth has been restricted
to the oceans, where the risk of exposure to ultraviolet radiation is greatly reduced.
But now, atmospheric oxygen levels are high enough to form a protective ozone layer
over the planet. This shield is a critical factor in the emergence and survival of
complex life on land.

Vendian extinction

Date:

543 mya

Intensity:

3

Affected:

Some single-celled algae and
soft-bodied animals go extinct

Hypotheses:

Sea-level changes, oxygen depletion

Summary:

Single-celled algae called acritarchs
and the Ediacaran animals vanish from the fossil record at the end of the Vendian, the
period that closes the Proterozoic era. If they were wiped out by an extinction event,
changes in sea level or lowered oxygen levels are likely to blame. But if, as most
scientists think, the soft-bodied Ediacarans are actually ancestors of Cambrian
faunas, their disappearance in the fossil record may simply demonstrate the
incompleteness of this record.

An estimated 250,000 fossil species have been recorded to date,
which is only about one percent of the 4 billion animal and plant species thought to
have existed over the past 600 million years or so. A high percentage of these --
perhaps 95 percent -- are hard-shelled marine creatures. For all that we have learned
about evolution from the fossil record, we know very little, relatively speaking,
especially of soft-bodied animals, which generally do not preserve as well.